You can also check the following:You should begin by checking the following:

Your modem and/or router are both on and are not indicating errors. If programs other than Firefox are also having problems connecting, the problem may be with your Internet Service Provider (ISP). Contact your ISP for more information.

If you are using a wireless connection, make sure that you are using the correct access point.

Your Domain Name System (DNS) server translates web addresses (URLs) into IP addresses and is usually provided by your ISP. If the DNS server is overloaded, you may be unable to view some or all websites. For example, if http://74.125.53.99 or http://74.125.159.105 work but http://www.google.com doesn't, you may have a DNS problem. You can use an alternative service such as OpenDNS or Google Public DNS.